Answers (3)
It's religious.
Genesis 3:17 And unto Adam he said, Because thou hast hearkened unto the voice of thy wife, and hast eaten of the tree, of which I commanded thee, saying, Thou shalt not eat of it: cursed is the ground for thy sake; in sorrow shalt thou eat of it all the days of thy life;
So wash that cursed ground off your hands before you eat.
Think of all the things you touch in a single day.
Now, think of all the other people that might have touched those things in a day.
Next, think of what all of those people that might have touched those things, and what they've touched in a day.
Odds are, there've been a few people that touched feces and/or urine, other bodily fluids, garbage, dirt, allergens, etc.
You don't want that going in your mouth all because you didn't wash your hands.